The present invention relates to composite electrodes having improved electrical performances along with good mechanical properties and resistance to aggressive reagents currently being used in the course of electrochemical processes.
Electrochemical processes take place at the surface of electrodes. A very fundamental concept in electrochemistry is the overvoltage or electrical overpotential that must be applied between two electrodes to generate a given electrochemical reaction. The potential of an electrode having a zero-current value is named the equilibrium or standard potential (E.sub.o). When practical applications are contemplated for an electrochemical process, the desired end compound can be produced by the application of a high current density. The voltage that must be applied accross the cell to induce useful values of current through the cell is substantially higher than the equilibrium potential. The difference between the actual potential at high current density and the equilibrium potential is designated as the overpotential. Depending on the system used and the intensity of the current, the overpotential can vary from a few millivolts to several hundreds of millivolts of the equilibrium poential.
It will be readily understood that the existence of an overpotential represents an energy loss and great efforts have been applied to reduce as much as possible the voltage above the standard potential E.sub.o which is required to drive an electrochemical system at a practical rate.
The existence of an overpotential is a complex phenomenon but has been noted that the overpotential generally increases as the current density, that is the number of amperes per square centimeter, increases. An obvious measure to decrease the overpotential would therefore consist in increasing the surface of the electrode in order to reduce this current density. Divided particles can be generated with high specific surfaces. If a section of one square cm on an electrode is covered with fine conducting particles, the actual surface offered to the reaction can be as high as one square meter, thus increasing the available surface by a factor of 10,000. Although not all of this surface is available for electrochemical processes, there is a substantial gain from the polished surface. A roughness on the surface of the electrode, such roughness being defined as the ratio of its actual surface to its geometric area, is therefore a good way of decreasing the overpotential by reduction of the current density.
There are many techniques which are well known for preparing electrodes having high specific surfaces. Amongst the known techniques, there may be mentioned plasma-spray, sputtering of metallic powders, electrodeposition and codeposition of material that can later be leached out of the catalyst, gas diffusion electrodes, sintering of metallic powders coated on a substrate and use of a packed-bed flow reactor with metallic powders.
Notwithstanding that the prior art does teach the preparation of high surface electrodes, the prior art procedures also require a fairly large amount of binder when electrodes are shaped from particulate matter, which results in a decrease of surface area with the consequence that an increase in electrical resistance is the end result.
Accordingly, it would be highly desirable if high surface electrodes could be devised whereby the use of the binder of particulate substrate would be reduced to a minimum thus increasing the available surface area for a given amount of material with corresponding advantages, the main one being a substantial reduction in electrical resistance due to an increase in conductivity.